1. Field of Invention
The present invention relates to a display device and a control method thereof.
2. Description of the Related Art
A display device such as a liquid crystal display (LCD) includes a liquid crystal panel including a liquid crystal layer and displaying an image, a backlight unit to emit light on the liquid crystal panel and an inverter to drive the backlight unit.
The liquid crystal display includes a plurality of switching elements such as thin film transistors (TFTs) including semiconductor made of amorphous silicon (a-Si), a plurality of pixel electrodes, and a common electrode.
The amorphous silicon (a-Si) is sensitive to light. That is, a-Si TFT becomes conductive and its resistance is reduced when it receives light. When the light is removed, the a-Si TFT becomes semi-conductive and its resistance rises relatively to allow the thin film transistors to be affected by charging voltages of liquid crystal capacitors formed between the pixel electrodes and the common electrode. When light is emitted on an a-Si TFT the parasitic capacity of data lines is increased and a screen noise is created.
When the backlight unit emits light consistently to the liquid crystal panel, a liquid crystal panel receives light uniformly, which does not trigger any problem. However, a problem arises when brightness of the backlight unit is adjusted by an inverter driving signal generated by PWM (pulse-width modulation) which turns the backlight unit on and off periodically to improve display quality.
When a frequency ratio of a synchronous signal such as a vertical synchronous signal or a horizontal synchronous signal for controlling image display on the liquid crystal display does not coincide with the inverter driving signal, which is a PWM signal, regular movement of lines are observed in each frame, causing waterfall noise.
Thus, display devices have recently employed a synchronous inverter to synchronize the frequency of the synchronous signal and the frequency of the inverter driving signal, at a proper ratio to minimize such a noise.
At this time, magnitude of each high level section and each low level section of the inverter driving signal may be defined by using the number of clocks of a clock signal such as a main clock signal for being supplied with image signals corresponding each pixel of the liquid crystal display from an external or a data clock signal for applying to the image signals to corresponding pixels.
In more detail, when the inverter driving signal is generated in synchronization with the horizontal synchronous signal, the magnitude of the sections of the inverter driving signal is defined based on the number of clocks of the clock signal for pixels of one pixel row. When the inverter driving signal is generated in synchronization with the vertical synchronous signal, the magnitude of the sections of the inverter driving signal is defined based on the number of clocks of the clock signal for pixels of the number of predetermined pixel rows.
However, in the synchronization of the synchronous signal and the inverter driving signal in the conventional display device, errors are excessively generated such that a flicking error, etc. occurs.
Particularly, when the inverter driving signal is generated in synchronization with the horizontal synchronous signal, much noise due to the asynchronization does not create because the number of pixels with synchronous relationship with the inverter driving signal is not many such the pixels of one pixel row. However, when the inverter driving signal is generated in synchronization with the vertical synchronous signal, the number of pixels of synchronous relationship with the inverter driving signal is many such (the number of pixel rows×the number of pixels for one pixel), even if one of all pixel rows is asynchronous, the number of pixels having the asynchronous relationship with the inverter driving signal largely increases. Thereby images are shaky and flicker occurs.